Air Handling Unit

AIR HANDLING UNIT:

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General Description Operation Technical Specifications

General Description The air handling unit is an integrated piece of equipment consisting of fans, heating and cooling coils, air-control dampers, filters and silencers. The purpose of this equipment is to collect and mix outdoor air with that returning from the building space. The air mixture is then cooled or heated, after which it is discharged into the building space through a duct system made up of five-feet diameter pipes.

Click for detailed diagram 1. Collection and Mixing of Outdoor Air and Air from the Building 2. Heating and Cooling System for the Air Handling Unit 1. Heating System 1. Steam Heated Coil 2. Baseboard Radiators 3. Reheat Coils 4. Solar Radiation and Heat Generated by the Equipments and the Building's

Occupants 2. Cooling System 1. Direct Evaporative Cooling 2. Indirect Cooling 3. Economiser or Free Cooling

Collection and Mixing of Outdoor Air and Air from the Building The centrifugal return fan pulls air from occupied building space (72 Return.Air.Temperature) through the return air ducts. The return fan has airfoil type blades which are the most efficient among all centrifugal fan blades. Some of this air is exhausted outside through the exhaust air damper, while a small amount of it (variable) continues through the air handling unit to mix with air drawn in from outdoors. This mixture of outside air and return air, before additional heating and cooling, is called mixed air (71 Mixed.Air.Temperature). The mixed air is filtered before entering the supply fan. The airfoil type centrifugal supply fan pushes the air through the heating and cooling stages of the AHU. The air is then distributed through a system of ductwork to all areas of the building.

Heating and Cooling System for the Air Handling Unit (AHU) Heating System The main energy source for heating the ITLL is the main power plant on campus. Burning natural gas to boil water, the plant provides steam to the ITLL building. The heating coil in the AHU uses steam made by a heat exchanger in the ITLL mechanical room.

The ITLL building is heated in different ways: the steam heated coil in the main AHU, the baseboard radiators, reheat coils in the ducts, solar radiation and building equipment and occupants. a. Steam Heated Coil b. Baseboard Radiators c. Reheat Coils. d. Solar Radiation and Heat Generated by Building Equipment and Occupants

The Steam Heated Coil in the Main Air Handling Unit (AHU) Once the cold season is in full swing, steam is supplied to the large steam coil in the AHU. This coil heats the building supply air leaving the AHU, sending warm air to all of the rooms, largely eliminating the need for the reheat coils. This coil uses a tube-in-tube arrangement to prevent freezing in case the steam valve is off and cold outside air is passing through. The coil consists of a copper header supplying steam to a copper tube which passes through a continuous aluminium fin for added heat transfer surface area. Due to the high temperature of steam relative to the air it is heating (approximately 180°F or 82.22°C) steam vs. approximately 70°F or 21.11°C air), the steam coil only needs to be of the single-pass type to accomplish the required heat transfer to the air. The coils are installed in a staggered arrangement, allowing for easy removal and maintenance.

Baseboard Radiators Heating coils using hot water add heat to individual rooms, as required. As outside temperatures drop in the fall, indoor temperatures also drop, beginning with rooms having exterior walls and windows. The thermostats in these rooms (e.g. Room 160) sense the drop in temperature and instruct the control system to turn on the hot water to the baseboard radiators and heating coils in the air supply ducts (reheat coils in the VAV boxes) supplying those rooms with heat. The primary function of the baseboard radiators is to prevent cold drafts from exterior walls and windows.

Reheat Coils In the early fall, before the cooling system has been shut down for the season, cool air is still being supplied to the rooms. The reheat coils re-heat the air before it enters rooms which may require heating. This way, other rooms in the building with high heat gain from equipment and occupants can continue to be cooled. It may seem like a waste of energy to cool the air in the AHU and then reheat it, but outside air from outside must be continuously supplied during occupied hours to maintain acceptable air quality.

Solar Radiation and Heat Generated by Building Occupants Solar gain through the large, south-facing windows also provides some heating. In Colorado's relatively sunny climate, the winter solar heat gain outweighs the heat loss, making this an

energy-saving design. The windows are shaded and specially coated to prevent unwanted heat gain in the summer. Rooms with more people and equipment require less additional heat to keep the room warm. Individual thermostats (e.g. Room 160) detect this temperature rise and reduce the heating to such rooms.

Cooling System The ITLL building has two stages of cooling in the main AHU: direct evaporative cooling and indirect cooling, which operate in conjunction to provide cool supply air at the lowest energy cost. In addition, economizer, or free cooling (see below), is used when conditions allow. All through the cold season, the AHU supplies air of a constant temperature (66 Air.Handling.Unit.Discharge.Air.Temperature) to the whole building (about 55&3186;F or 12.77°C). Although there is no local cooling for any of the rooms except the computer rooms, the variable air volume (VAV) system allows different amounts of cooling in each zone (rooms or areas sharing a single thermostat). a. Direct Evaporative Cooling b. Indirect Cooling c. Economizer or Free Cooling

Direct Evaporative Cooling Because of the generally low humidity (76 Outside.Air.Humidity.1) in Colorado, direct evaporative cooling is a very economical way to cool air. As water evaporates into the air, the air temperature drops (heat energy from the air is used to change the water from liquid to vapor) and the air is humidified. Depending on how much cooling is required, from one to three stages of evaporative cooling may be used. Because evaporative cooling increases the humidity of the air, this process requires low outside air humidity and that humidified building air (77 Return.Air.Humidity) be continuously exhausted.

Indirect Cooling When the outside air humidity (76 Outside.Air.Humidity.1) is too high to allow direct evaporative cooling, the indirect cooling stage is automatically activated. This stage consists of an outdoor cooling tower (fluid cooler FC-1) which provides cold glycol solution to a cooling coil (a tube with fins attached) in the AHU. This coil has copper headers and tubes passing through a continuous aluminium fin for added heat transfer area. Because the water-glycol solution passing through the cooling coil is not considerably colder than the air it is cooling (approximately 50&3186;F or 10°C) solution vs. approximately 70°F or 21.11°C) air), the tubes in the cooling coil must make several passes (about 6) to accomplish the required heat transfer

from the air. The coils are installed in a staggered arrangement, allowing for easy removal and maintenance. A drip pan piped to drain below the indirect cooling coil carries away any moisture which may collect on the coil when the air is dehumidified. The fluid cooler also requires low outside air humidity to function, since it cools the glycol solution through evaporation of water on the outside of its cooling coil. Because of this, these systems are not used in climates where high humidity conditions occur.

Economizer or Free Cooling During certain times of the year, the interior of the building requires cooling even though the outside air temperature (99 Outside.Air.Temperature.1) is relatively low (less than 55.4°F or 13°C). In this case, both the outside and exhaust air dampers are fully opened (80 Exhaust.Damper) and the building is cooled for "free." Since the fan power required (100 Supply.Fan.Watts) to move the air is the only energy cost, this is truly an economical way to cool the building. This 100% ventilation is equivalent to opening all the windows.

Operations The main air handler unit serving the ITLL building is of the variable air volume (VAV) type. This means that as the overall building airflow requirements increase or decrease, the main fans in the air handler speed up and slow down, to provide only as much air as is required. Due in part to special variable speed motor controllers used to control the fan speeds, the initial costs of installing a VAV system are somewhat higher than a traditional constant volume system. However, the money saved when these big fans are running below maximum speed quickly pays off. The pre-programmed Direct Digital Control (DDC) system operates the equipment differently during occupied and unoccupied periods. This is another energy-saving strategy which allows the air conditioning equipment to "rest" during periods when the building is unoccupied. Many sensors installed in the air handler give the DDC system the information it needs to control the fans, dampers, etc. a. System Operation during Occupied Periods b. System Operation during Unoccupied Periods c. General System Operation during All Modes

System Operation During Occupied Periods

The (adjustable) discharge air temperature setpoint is reset based on the outside air temperature (99 Outside.Air.Temperature.1) (OAT) according to the following schedule: For an OAT of 55°F (12.77°C), the discharge air temperature is 55°F (12.77°C). For an OAT of 20°F (-6.66°C), the discharge air temperature is 65°F (18.33°C).

Supply Fan The supply fan speed is controlled to maintain 1.00 in. WG (inches of water gage) static pressure (103 Duct.Static.Pressure) in the supply duct.

Return Fan Whenever the supply fan is running, the return fan also operates (113 Return.Fan.Status.2) and its speed (86 Return.Air.Flow.Rate) is controlled to maintain 0.10 in. static pressure (109 Return.Static) in the return air plenum.

Exhaust Air Damper The damper position (80 Exhaust.Damper) is adjusted to maintain a static pressure (102 Building.Static.Pressure) of 0.05 in WG in the occupied building space. This helps to minimize infiltration into the building.

Outside Air Dampers Based on a signal from the outside air flow sensor, the DDC system controls the outside air and return air dampers to maintain a minimum outside air flow (85 Outside.Air.Flow.Rate) of 8700 cfm (cubic feet per minute). If too little outside air is mixed with the return air, the indoor air quality can become unacceptable. The minimum air flow rate prevents this from happening.

Outside Air Economizer An additional energy saving feature of the AHU uses outside air, instead of a cooled mixture of outside air and return air, to cool the building. This is called an "economizer cycle," or "free cooling" and would operate, for example, when the building requires cooling while the (99 Outside.Air.Temperature.1) is below 55°F (12.77°C). This allows the cooling and heating coils to be turned off, while the fans pull in outside air. When the outside air economizer is being used, the outside and return air dampers operate on their own control loop to maintain the discharge air temperature (66 Air.Handling.Unit.Discharge.Air.Temperature). That is, the dampers are opened or closed in the proper ratio to provide a mixed air temperature (71 Mixed.Air.Temperature) sufficient to cool the building and to provide acceptable indoor air quality; no cooling is supplied by the coil in the AHU.

It is virtually impossible to have a situation that when the building requires cooling on a hot day, the economizer heating activates; this is not, therefore, part of the control system programming.

Steam Heating Coil The valve status controlling the steam supply to the heating coil is automatically set to maintain the heating coil discharge air temperature setpoint. The heating coil discharge air temperature is reset through a cascade action based on the AHU discharge air temperature setpoint. That is, actions are taken one at a time to maintain the discharge temperature, instead of simultaneously. For example, the return air damper might first be opened. If bringing in more warm air from within the building and mixing it with outside air is not enough to raise the AHU discharge air temperature (66 Air.Handling.Unit.Discharge.Air.Temperature), then the steam valve is opened more.

Indirect Cooling Coil For outside air temperatures above 55°F (12.77°C), the water pump continuously circulates chilled glycol solution through the cooling coil, while the DDC system controls the three-way (adjustable bypass) valve to maintain the required cooling coil discharge air temperature (66 Air.Handling.Unit.Discharge.Air.Temperature). If this temperature drops below 50°F(10°C), the bypass valve is opened to prevent further cooling and potential freezing of the cooling coils.

Direct Evaporative Cooling Section This section consists of a two honeycomb type porous pads (4 in. and 8 in. thick) over which water is sprayed by the two sump pumps. When air is blown through the holes, some of the water evaporates and cools the air stream in the same way sweating cools the body on a breezy day.

Sump Schedule If the outside air temperature drops below 40°F (4.44°C), the water sump is drained; it is filled when the outside air temperature rises above 55°F (12.77°C), and the evaporative cooler is needed to meet the cooling requirements. The sump is drained on Sunday mornings, if it has not been drained for the previous 4 days, while a daily 60-minute pad dry-out period runs from 5 a.m. to 6 a.m. if the cooler has not been off for at least one hour in the last 24 hours.

Sump Pumps The sump pumps cycle in sequence (pump #1 to 4 in. pad, pump 2 to 8 in. pad, and both pumps on) to maintain the AHU air discharge temperature setpoint; minimum on-off cycle timers prevent the pumps from short-cycling. To allow the evaporative cooler discharge temperature to stabilize, a 10-minute time delay between pump stages is used. The pumps only operate if the outside air damper is completely open, the supply fan is still running and the sump has been filled.

Humidity Control

If the return air relative humidity (77 Return.Air.Humidity) exceeds 65% for 30 minutes, the highest current stage of sump pump operation is shut off until the relative humidity drops below 60%.

Outside Air Damper Control As the mixed air temperature (71 Mixed.Air.Temperature) drops, the outside air damper is closed until the fraction of outside air is at the minimum to maintain acceptable indoor air quality (typically a 20% fraction of outside air). If the mixed air temperature drops below 40°F (4.44°C), the outside air damper is closed further (below its minimum opening) to prevent freezing of the AHU components.

System Operation During Unoccupied Periods Supply and Return Fans, Dampers and Valves The fans are de-energized, the outside air and exhaust air dampers are closed and the steam coil control valve is opened to the coil.

Baseboard Radiators The radiation heating valves are controlled to maintain a temperature of 65°F (18.33°C) in each zone. If this temperature drops below 63°F or 17.22°C (with a 5°F or 2.77°C deadband), the AHU cycles on to maintain the zone temperature using supply air at a maximum temperature of 85°F (29.44°C).

General System Operation During All Modes Freeze Protection A thermostat stops the supply fan, starts the return fan, starts the condenser water pump, opens the control valves to the heating coil and indirect cooling coil and closes the outside air damper if it senses an outside air temperature below 40°F (4.44°C). Glycol solution is circulated in the cooling coil during this time to prevent the solution from freezing in the coil.

Smoke Detectors Smoke detectors located at the supply duct in the air handling unit act to stop the supply and return fans and close the outside air, exhaust air and smoke dampers if smoke is detected. These detectors are integrated with the fire alarm system.

Duct Pressure Control The duct static pressure sensor controls the supply fan speed to maintain the duct static pressure setpoint. If excess static pressure (103 Duct.Static.Pressure) is detected, the supply and return fans are shut down by a duct high pressure limit switch.

Technical Specifications

Air Handler System - Exhaust Air Damper Plan Symbol Submittal Ref. # Location Manufacturer Supplier Series Size Blade Type Area Factor

AHU-1 15855 I-09 Industrial Acoustics Company Engineered 752-4411 CD-50 Width: 150 in, Ht: 36 in Airfoil shape, 3" width 0.11

Air Handler System - Heating and Cooling Coil Manufacturer

Engineered Air Inc.

Supplier

Engineered 752-4411

Model No.

LM-48-C

Service

Heating

Tube Material

Copper

Outer Tube Size

5/8 in. outside dia.; .035 in. thick

Fin Material

Aluminium

Fin Thickness

0.0065 in.

Number of Coils 4 STEAM COIL DATA: Following data is for 1 coil @ 5400 ft elev. Type Non-freeze Size Width: 42 in, Height: 60 in Number of Rows 1 Face Area 17.5 sq. ft Design Air Volume 12075 cfm Air Velocity 690 fpm Air Friction 0.13 in H2O Capacity 575629.74 Btu / hr Entering Dry Bulb 56°F Leaving Dry Bulb 100.14°F Latent Heat 966.5 Btu / lb Steam Flow 596 lb / hr Steam Pressure 2 psig Steam Temperature 218°F Ent. T.D. 162°F (adjustable) COOLING COIL DATA: Following data is for 1 coil @ 5400 ft elev. Service Cooling Number of Coils 4 Type Ultrafin Size 48 in x 72 in x 6 Rows / 12 FPI Pass-Circuits-Blank 12 - 16 - 0 Left Hand (looking in the direction Return Side of airflow) Supply Side Left Hand Total Capacity 265110 Btu / hr Sensible Capacity 265110 Btu / hr 12075 cfm (actual air at coil leaving Air Flow side) Air EDBT 95.00°F Air EWBT 63.00°F Air LDBT 69.79°F

Air LWBT Moisture Leaving Air Velocity FLUID Entering Temp. Leaving Temp. Water Flow 1. Water Velocity 2. Water Press. Drop 3. Drain Pan Depth

54.81°F 0.00 lb / hr 503 fpm 35% Ethelyne-glycol 68.00°F 74.92°F 85.00 gpm 5.80 ft / sec 29.97 ft. WC 6 in

Air Handler System - Evaporative Cooling Medium Plan Symbol Submittal Ref. # Location Manufacturer Supplier Free Face Size Face Velocity Evap. Cooling Efficiency Water Flow to 4" Media Water Flow to 8" Media

AHU-1 15855 I-09 Engineered Air Engineered Air 752-4411 156 in x 86 in 518 ft / min 89 % 7.8 gpm (approx.) 15.6 gpm (approx.)

Air Handler System - Pleated Air Prefilter Plan Symbol Submittal Ref. # Location Manufacturer

AHU-1 15855 I-09 Engineered Air

Supplier Series Size

Engineered Air 752-4411 LM 156 in x 86 in

Air Handler System - Outside Air Damper Plan Symbol Submittal Ref. # Location Manufacturer Supplier Series Size Blade Type Blade Action

AHU-1 15855 I-09 Industrial Acoustics Company Engineered Air 752-4411 CD-403 Width: 150-1/4 in, Ht: 36-1/4 in Airfoil shape, 3" width 35% Opposed Blade

Air Handler System - Return Air Damper Plan Symbol Submittal Ref. # Location Manufacturer Supplier Series Size Blade Type Blade Action

AHU-1 15855 I-09 Industrial Acoustics Company Engineered Air 752-4411 CD-403 Width: 150-1/4 in, Ht, 24-1/4 in Airfoil shape, 3" width 8% Opposed Blade

Air Handler System - Return Fan Plan Symbol

AHU-1

Submittal Ref. # Location Manufacturer Supplier Model Elevation Airstream Temp Startup Temp Volume Static Pressure Tip Speed Output Velocity Fan Speed Fan Rotation Fan Power Motor Power Motor Voltage Motor Speed Motor Efficiency

15855 I-09 Greenheck Engneered Air Inc. 752-4411 54-PLN 5400 ft 70°F 70°F 40300 cfm 3.270 in WC 10297 ft / min 2382 ft / min 725 rpm CW (viewed from motor drive side) 30.3 Hp 40 Hp 460 V, 3 ph 1725 rpm 93.6 %

Air Handler System - Supply Fan Plan Symbol Submittal Ref. # Location Manufacturer Supplier Model No. Model Elevation Airstream Temp Startup Temp Volume Static Pressure Tip Speed

AHU-1 15855 I-09 Greenheck Engneered Air Inc. 752-4411 LM-48-C 54-PLN 5400 ft 70°F 70°F 48300 cfm 4.940 in WC 12555 ft / min

Output Velocity Fan Speed Fan Rotation Fan Power Motor Power Motor Voltage Motor Speed Motor Efficiency

2855 ft / min 884 rpm CW (viewed from motor drive side) 54.7 Hp 60 Hp 460 V, 3 ph 1725 rpm 93.6 %